1. Field of the Invention
The present invention relates to a method for producing a glass preform for use in the fabrication of an optical fiber. More particularly, the present invention relates to a method for producing a fluorine-containing glass preform in which fluorine is selectively added to a cladding part by the VAD (vapor phase axial deposition) method, particularly to improvement of a dehydration step.
The method according to the present invention is advantageous for producing a glass preform to be used in the fabrication of an optical fiber which is used in long distance optical communication or in a radiation filled atmosphere.
2. Description of the Related Art
Generally, an optical fiber comprises a core through which light propagates and a cladding which surrounds the core and has a refractive index lower than that of the core. In a quartz (SiO.sub.2) glass base optical fiber, a refractive index difference between the core and the cladding is formed by the addition of an additive which increases the refractive index of the glass to the core and/or by the addition of an additive which lowers the refractive index of the glass to the cladding.
Fluorine is an additive which can lower the refractive index of the quartz glass without adversely affecting transmissionlose characteristics of the optical fiber. When the additive which increases the refractive index (e.g. GeO.sub.2) is added to the core, transmission loss due to Rayleigh scattering or transmission loss in the presence of radiation is increased. Therefore, it has been tried to produce an optical fiber in which an amount of the refractive index-increasing additive to be added to the core is decreased, ideally to zero while decreasing the refractive index of the cladding by the addition of fluorine to the cladding, and various methods for producing such optical fiber have been proposed.
One of such proposed methods comprises forming a porous glass preform (soot preform) comprising a core part and a cladding part which correspond to the core and the cladding of the optical fiber, respectively by using a plural number of burners for synthesizing glass soot, subjecting the soot preform to a fluorine-addition treatment such as heating of the soot preform in an atmosphere comprising a fluorine-containing compound to add fluorine to the cladding part, and then heating the soot preform at a high temperature to make it transparent to obtain a transparent glass preform consisting of a core part made of SiO.sub.2 glass and a cladding part made of a fluorine-added SiO.sub.2 glass. In this method, since both the core and cladding parts are porous, fluorine tends to be uniformly added to both parts, so that it is very difficult to selectively add fluorine to the cladding part.
In another proposed method, as shown in FIG. 1, a burner 106 for sintering the core part is provided between a burner 104 for forming the core part and a burner 105 for forming the cladding part, and immediately after a porous glass body of the core part 102 is synthesized around a peripheral surface of a starting glass rod 101 by the burner 104, it is heated by the burner 106 to increase its bulk density. Thereafter, a porous glass body of the cladding part 103 is synthesized on the core part by the burner 105 to obtain a porous preform in which a density of the core part has been suitably adjusted. Thereby, during the addition of fluorine to the cladding part, diffusion and addition of fluorine to the core part are prevented, so that a glass preform having desired refractive index profile is produced (cf. Hanawa, et al, "Fabrication of Pure Silica Core Fibers by VAD Method", Denkitsushin Gakkai Ronbunshi, Vol. J68-C, No. 8, 597-604 (1985)). In FIG. 1, an upper arrow and a circular arrow indicate directions of pulling up and rotating the starting member, respectively. By the way, the addition of fluorine to the porous glass body closely relates to the bulk density of the porous glass body. The bulk density at which fluorine is easily added is said to be at most about 0.5 g/cm.sup.3. By this method, a bulk density of the surface part of the core part glass body but not of the whole core part is increased up to bout 2.0 g/cm.sup.3. Therefore, the addition of fluorine into the core part is prevented by the surface part having the large bulk density.
According to the above described method in which the bulk density of the surface portion of the core part porous glass body is adjusted, the core-cladding porous glass body can be produced in one step, and the addition of fluorine to the core part is prevented. However, a dehydration gas such as chlorine-containing gas hardly penetrates into the core part, and the dehydration gas cannot reach the center portion of the core part.
Light absorption caused by OH groups due to residual water in the glass has a peak around a wavelength of 1.39 .mu.m, it has influences on light transmission in a 1.30 or 1.55 .mu.m wavelength band and results in increase of transmission loss. Therefore, dehydration of the core part is very important.
In the above described method, after the formation of the core-cladding porous glass body, dehydration is effected and then fluorine is added. To decrease the amount of the OH groups to such a level that the transmission loss at a wavelength of 1.39 .mu.m is decreased to 10 dB/km or less, namely the light transmission in a 1.55 .mu.m band is not influenced, dehydration should be continued for 200 hours or longer (see page 602, FIG. 12 of Hanawa et al, supra.).
In contrast to the other conventional methods for producing the glass preform in which the dehydration is completed in about 2 hours and he OH group content can be reduced to a level at which the transmission loss at the wavelength of 1.39 .mu.m is 1 to 2 dB/km, the above described method has poor productivity and is not practically employed.